1. Field of the Invention
The present invention relates to a semiconductor laser module and a method for manufacturing the laser module.
2. Related Prior Art
A Peltier device has been used to be installed within the laser module to cool down the laser diode (hereinafter denoted as LD). The Japanese patent published as JP-2002-094171A has disclosed a method for manufacturing an optical module that installs a laser diode and a Peltier device with a small-sized package. The method disclosed in the prior patent applies a soldering to install the laser diode on the Peltier device.
Another Japanese patent published as JP-2003-158328A has disclosed a method for assembling the laser diode on the Peltier device, in which a collet that handles the LD chip provides a heating means to raise the temperature of the solder between the LD chip and the chip carrier for the LD. Still another patent published as JP-2002-076501A and U.S. Pat. No. 6,506,624, have disclosed a method for assembling the Peltier device, in which a bottom of the Peltier device is fixed with a solder, types of which is PbSn, while the upper plate thereof mounts the LD chip with a solder of BiSn. Because the BiSn may be easily oxidized, the process using the BiSn solder is carried out within a hydrogen atmosphere. However, recent electronic equipments are strongly requested that a solder without lead and its compounds, the use of PbSn solder should be avoided. Not only a butterfly package with the box shape but a co-axial package with the. cylindrical shape comes to install the Peltier device therein. Japanese patent published as JP-2004-253779A and a conference paper by Atsushi Miki et. al., titled by “Extremely Compact DFB Laser Module for the DWDM and CWDM Systems in the Wide Temperature Range”, ECOC 2004, Th. 2. 4. 4, have disclosed such laser module with the co-axial package.
As commented, a trend to use a lead free solder in electronic equipments has recently and globally accelerated. Conventionally, when a LD chip is mounted on the chip carrier with the good heat conductivity, the PbSn solder, whose melting point is about 180° C., can be used. However, the above trend to avoid the lead compound may hesitate to use this solder. Although a first substitution may be SnZn whose melting point is about 200° C., this increase of 20° C. of the melting point causes various subjects to be solved.
The Peltier device generally causes a large temperature difference between the upper and lower plates thereof because the material used is one type of thermal insulator. Rather, a superior Peltier device makes the larger temperature difference between the plates. Due to this temperature difference, a type of solder applicable in the assembly of the laser module is restricted. That is, in a case that the Au0.8Sn0.2 with a melting point of about 280° C. is applied for fixing the Peltier device in the lower plate thereof to the base, while the LD is fixed to the LD carrier with a SnZn solder, which does not contain the lead (Pb) and the. melting point is about 200° C., to fix the LD carrier on the upper plate of the Peltier device is restricted to use a solder whose melting point between that of the Au0.8Sn0.2 and the SnZn. One typical example for such solder may be Au0.1Sn0.9 and SnSb with the melting point of 220° C. and 240° C., respectively.
However, in the case that the SnSb solder (melting point: 240° C.) is used between the upper plate of the Peltier device and the LD carrier, the temperature of the base and the lower plate of the Peltier device is necessary to be raised nearly to the melting point of the Au0.8Sn0.2 solder laid therebetween in order to heat up the upper plate of the device to the melting temperature of SnSb, 240° C., to solder the LD carrier on the upper plate of the Peltier device due to the thermally insulating characteristic inherently attributed to the Peltier device. In such case, the softening of the Au0.8Sn0.2 solder may occur to cause a misalignment between the Peltier device and the base that affects the optical coupling between the LD and the optical fiber.
Another solder with a lower melting points, such as Au0.1Sn0.9 (melting point: 220° C.), is applied may solve the subject above mentioned that the softening of the first solder between the base and the Peltier device. However, this case may not assure a temperature difference between the solder, the SnZn (the melting point: 200° C.), between the LD carrier and the LD chip, so the wettability of the upper surface of the LD carrier, where the LD is to be mounted thereon, may not be secured and degrade the reliability of the assembly.
For the process of heating the collet disclosed in the JP-2003-158328A, although this process may be escaped from the restriction of the solder and increase the variation of the soldering process, the equipment used in the process becomes complicated to enhance the process cost. Moreover, processes disclosed in prior arts listed above are necessary to solder twice, that is, the first soldering between the base and the lower plate of the Peltier device and the second soldering between the upper plate of the device and the LD carrier after the first soldering, and both soldering accompany with the rapid increase and decrease of the temperature which degrades the performance of the Peltier device and the reliability thereof.
The present invention is performed to solve the above subject, that is, even when the SnZn solder as the lead free material is applied for the fixing of the LD to the LD carrier, the invention may relax the restriction of the solder used for fixing the Peltier device to the base and the LD carrier to the Peltier device and may prevent the Peltier device from degrading its performance and reliability by the heat cycle.